Turbine entry gas pressures have risen continuously recently, and these rises affect the secondary or cooling air system. In particular, High Pressure (HP) turbine blades generally require `showerhead` cooling, thereby necessitating the cooling air feed pressure to be greater than the turbine entry gas pressure. A consequence of these pressures is that a rudimentary air seal between HP nozzle guide vanes and the HP turbine vanes is no longer acceptable because the leakages around the seal are unacceptably high.
In addition to the high turbine entry temperatures and high disc speeds, there is a constant necessity to improve efficiency, for example, by increasing the thrust/weight ratio.
There is therefore a move towards omitting certain non-fundamental components such as coverplates. At the same time, however, non-useful leakages must be reduced because the secondary air available for cooling crucial components such as the flame tube, HP nozzle guide vanes and HP turbine blades will become reduced, for example, to achieve certain smoke levels.